Resistance to a widely used flu drug is spreading among strains of the bird flu virus in Asia, according to a group of US-based researchers, while another group warns that even less harmful strains of the virus can jump to humans.

A team led by Robert Webster at St Jude Children's Research Hospital in Memphis, United States, analysed genetic data from various strains of the bird flu virus collected in North America and South-East Asia between 1991 and 2004.

They found that H5 and H9 viruses resistant to amantadine, a drug used to treat human influenza, were becoming more common in Asia, especially in China.

By contrast, samples of the same viruses found in North America could still be treated with the drug.

A subtype of the H5 strain, known as H5N1, has killed 57 people in South-East Asia since late 2003. Tens of millions of chickens and other poultry have been destroyed in an effort to prevent the disease spreading.

Using human flu drugs to treat chickens in this way is raising concern among scientists, as it could favour the emergence of drug resistant viruses that are even harder to treat, either in birds or humans.

Although the H5N1 bird flu strain can infect people, no cases of person-to-person transmission have yet been confirmed. In theory, however, once inside a human body, the virus could swap genetic material with human flu strains, forming a 'superstrain' that would create a global influenza pandemic.

So far, scientists have been tracking only highly dangerous strains of the bird flu virus, such H5N1, for signs that they could mutate into a form that can spread rapidly between people.

But in separate research, Isabella Donatelli at the Istituto Superiore di Sanità in Rome, Italy, and colleagues have shown for the first time that less dangerous strains can also infect people.

In an article published online in The Journal of Infectious Diseases, Donatelli's team says that their findings show how important it is for infectious disease specialists to monitor outbreaks of even the less harmful bird flu strains.

The team, whose results came from tests in people working on poultry farms with bird flu outbreaks, advise that all poultry workers should be vaccinated routinely.

But an accompanying editorial — written by Frederick Hayden of the University of Virginia, United States and Alice Croisier of the World Health Organization – says that in developing countries where people have little access to vaccines, people should be better educated about food and water hygiene, as well as poultry handling. .